Fluoropolymers have properties such as extremely low coefficient of friction, wear and chemical resistance, dielectric strength, temperature resistance and various combinations of these properties that make fluoropolymers useful in numerous and diverse industries. For example, in the chemical process industry, fluoropolymers are used for lining vessels and piping. The biomedical industry has found fluoropolymers to be biocompatible and so have used them in the human body in the form of both implantable parts and devices with which to perform diagnostic and therapeutic procedures. In other applications, fluoropolymers have replaced asbestos and other high temperature materials. Wire jacketing is one such example. Automotive and aircraft bearings, seals, push-pull cables, belts and fuel lines, among other components, are now commonly made with a virgin or filled fluoropolymer component.
In order to take advantage of the properties of fluoropolymers, fluoropolymers often must be modified by decreasing their lubricity in order to be bonded to another material. That is because the chemical composition and resulting surface chemistry of fluoropolymers render them hydrophobic and therefore notoriously difficult to wet. Hydrophobic materials have little or no tendency to adsorb water and water tends to “bead” on their surfaces in discrete droplets. Hydrophobic materials possess low surface tension values and lack active groups in their surface chemistry for formation of “hydrogen-bonds” with water. In the natural state, fluoropolymers exhibit these hydrophobic characteristics, which requires surface modification to render it hydrophilic. The applications mentioned above all require the fluoropolymer to be modified.
One such modification includes chemically etching the fluoropolymers. For example, fluoropolymer films and sheets are often etched on one side to enable bonding it to the inside of steel tanks and piping; the outside diameter of small diameter, thin wall fluoropolymer tubing is etched to bond to an over-extrusion resulting in a fluoropolymer-lined guide catheter for medical use; fluoropolymer jacketed high-temperature wire is etched to allow the printing of a color stripe or other legend such as the gauge of the wire and/or the name of the manufacturer; fluoropolymer based printed circuit boards require etching to permit the metallization of throughholes creating conductive vertical paths between both sides of a double sided circuit board or connecting several circuits in a multilayer configuration.
The first commercially viable processes were chemical in nature and involved the reaction between sodium and the fluorine of the polymer. In time, some of the chemistry was changed to make the process less potentially explosive and hazardous, but the essential ingredient—sodium—remains the most reliable, readily available chemical ‘abrasive’ for members of the fluoropolymer family.
In addition to being hazardous, chemically etched fluoropolymer surfaces tend to lose bond strength over time. It has been shown that temperature, humidity and UV light have a detrimental effect on etched surfaces. Tests have shown that etched fluoropolymer parts exposed to 250° F. for 14 days exhibit bond strengths approximately 40% weaker than those done on the day they were etched. Further, depending upon the wavelength and intensity of the UV light source, the bond strength deterioration can occur over a period of months and years. It is thought that, due to the somewhat amorphous nature of these polymers, a rotational migration occurs over time, accelerated by some ambient conditions —especially heat—that re-exposes more of the original C2F4 molecule at the surface resulting in a lower coefficient of friction.
Another factor that is of concern with chemical etching of fluoropolymers is that of the depth of the etched layer. The sodium reaction with fluorine is a self-limiting one, and it has been shown to take place to a depth of only a few hundred to a few thousand Angstroms.